Abstract Adolescent Brain Cognitive Development (ABCD) is the largest long-term study of brain development and child health in the United States. ABCD consists of a Coordinating Center, a Data Analysis and Informatics Resource Center, and 21 research sites across the U.S. ABCD has enrolled a diverse sample of 11,878 9-10 year-olds, and is tracking their biological and behavioral development through adolescence into young adulthood. All participants receive repeated state-of-the-art neuroimaging, neuropsychological testing, bioassays, and detailed youth and parent assessments of substance use, mental health, physical health, and culture and environment. In March 2020, when our participants are ages 11-13, the world became substantially affected by the COVID- 19 pandemic, leading to an upheaval in the economy and the lives of almost every family. The majority of U.S. schools closed to reduce viral spread. Many parents incurred changes in work (from home, longer shifts, reduced wages, and/or job loss), some services and support systems became disrupted, and case counts and death tolls surge. The massive multifaceted impact of this unprecedented event has the potential to affect for decades those who are currently children. The proposed research immediately leverages the ABCD cohort, infrastructure, and existing protocol to rapidly characterize the impacts of the COVID-19 pandemic on each child in the study. By collecting this situational information as soon as possible, we can use existing ABCD data to examine perturbations in developmental trajectories of brain functioning, cognition, substance use, academic achievement, social functioning, and physical and mental health. The proposed project would query all ABCD participants and their parents multiple times about the impact of the pandemic on their lives and, in a subset of participants, examine their physical activity and sleep objectively with activity trackers (Fitbits), over the months of school closures, job loss, and disease spread. This will allow the consortium and scientific community at large to test multiple aims regarding how various facets of the pandemic affect development. This includes: (1) characterizing the impact of the COVID-19 pandemic on brain and cognitive development and onset of substance use; (2) evaluating the extent to which alternative schooling approaches exacerbate or mitigate the impact of the pandemic on brain and cognitive development and substance use outcomes; and (3) evaluating the extent to which family stressors exacerbate or mitigate the impact of the pandemic on neurobiological, cognitive, and substance use outcomes. This unprecedented crisis provides an opportunity to make use of ABCD's elaborate infrastructure and rigorous scientific processes to discern critical dimensions of development not previously envisioned.